CNG vs. LPG

CNG is Compressed Natural Gas, which is mainly methane compressed at a pressure of 200 to 248 bars. LPG is Liquefied Petroleum Gas, a mixture of propane and butane liquefied at 15 °C and a pressure of 1.7 - 7.5 bar. Some variants of LPG are primarily propane so LPG is often colloquially called propane. CNG is cheaper and cleaner, but LPG has a higher calorific value. Distribution is easier for natural gas over long distances via pipelines.

Production

Natural gas is created over hundreds of years deep within the earth. It has to be extracted (pumped to the surface) and cleaned to remove impurities including water to be marketable. The byproducts of natural gas processing are ethane, propane, butane, pentane and higher molecular weight hydrocarbons.

LPG is manufactured during the refining of petroleum (crude oil), or extracted from petroleum or natural gas streams as they emerge from the ground.

Cost

CNG is generally cheaper than LPG. However, propane produces over twice the amount of energy than natural gas. So LPG could be more cost effective than natural gas depending upon local prices (which tend to fluctuate).

Here's an NBC news story on how cost effective and convenient CNG is over gasoline:

Availability

There is an extensive distribution network for both CNG and LPG. But the retail availability of LPG is more compared to that of CNG. CNG distribution is mainly via pipeline networks that run from natural gas production wells to various distribution centers and finally into residential neighborhoods and commercial areas.

Energy Produced

LPG, vaporized and at atmospheric pressure, has a higher calorific value (94 MJ/m3 or 26.1kWh/m3) than natural gas (38 MJ/m3 or 10.6 kWh/m3). This LPG produces more energy than the equivalent amount of natural gas.

Safety and Environmental Impact

Although CNG does produce greenhouse gases upon combustion, it is a more environmentally clean alternative to other fossil fuels like gasoline or LPG. CNG is also safer than other fuels in the event of a spill because natural gas, being lighter than air, disperses quickly when released.

Substituting LPG for CNG

Because LPG produces more energy than CNG, it cannot simply be substituted for natural gas. Only gases that have the same Wobbe index are interchangeable. For LPG to have the same Wobbe index as CNG, it has to be mixed with air. A 60:40 ratio for LPG:air mixing is common but it depends upon the composition of LPG because LPG itself is a mixture of propane and butane. This mixture of LPG and air to substitute natural gas is called synthetic natural gas (SNG). Use of SNG is common before the CNG distribution network is fully operational.

Odorant

Ethanethiol or ethyl mercaptan is commonly added to both LPG and CNG. Mercaptan has a foul, rotten-egg smell that makes it easy to detect gas leaks.

Uses

CNG

Automobiles: Any vehicle running on gasoline can be converted to a bi-fuel vehicle (gasoline/CNG). It involves installing a CNG cylinder in the trunk, installing the plumbing system, installing CNG injunction system and the electronics. In the US, CNG vehicles have been introduced in a wide variety of commercial vehicles, from light duty trucks and sedans like taxi cabs to medium duty trucks like UPS delivery vans and postal vehicles to heavy duty vehicles like transit buses, street sweepers and school buses. In California, transit agency buses run on CNG. Coco Cola Enterprises delivered its beverages to London Olympics Games 2012 in bio methane fueled lorries.

Locomotives: Napa Wine Valley Train has been converted to be run with CNG. FerroCarril Central Andino in Peru has been running a freight line on CNG run locomotive since 2005. Iran, Pakistan, Argentina, Brazil and India have the highest number of CNG vehicles run.

LPG

Heating: LPG is used an alternative to heating oil and electricity in places where there is no natural gas pipe line.

Cooking: LPG is the most common cooking fuel in India and urban Brazil.

Refrigeration: Gas absorption refrigerators and air conditioning systems use LPG. But its use in motor vehicles for air conditioning has been discouraged due to the risk of fire.

Automobiles: A large number of vehicles, light, medium and heavy duty, around the world are fueled by LPG. There about 1,600 bi-fuel Ford F-150 pickup trucks in the California state fleet. Propane fueled forklifts are used both inside and outside warehouses and at construction sites.

The following video shows how to convert a car to use LPG along with gasoline.

Advantages of using CNG and LPG in vehicles

A CNG tank on the trunk of a CNG and gasoline powered Chevy Cavalier

CNG

Lead or benzene is not used. Hence lead fouling of spark plugs is eliminated.

Disadvantages of using CNG and LPG in vehicles

CNG

CNG occupies more space than gasoline. Additional space in the trunk of a car or the bed of a pickup truck is used to store the CNG tanks. This problem has been mitigated in factory-built CNG vehicles by installing the tanks under the vehicle body, e.g. Fiat Multipla, Fiat Panda, Volkswagen Touran Ecofuel, Volkswagen Caddy Ecofuel, and Chevvy Taxi.

LPG

LPG has lower energy density than gasoline or diesel and hence the equivalent fuel consumption is more.

LPG provides less upper cylinder valve lubrication. If a LPG-fueled engine is not suitable modified, it will lead to valve wear.

Comments: CNG vs LPG

Anonymous comments (4)

September 12, 2013, 4:30pm

The use of CNG in Delhi has not improved air qualtiy. Infact, the pollution lelvesl are alarming and Delhi is one amongst top pollutinh cities in the world. It is, therefore, necessary to look inot non polluting forms of fuel for the vehilces. Unfortunately, practice oriented researh using solar energy is getting aback seat. There is an urgent need to look into these issues.
Profesor Ashok Deshpande PhD (Environmental Engineering)

— 59.✗.✗.18

-1

October 18, 2013, 1:51pm

Under the safety section, both gases disperse into the atmosphere quickly with CNG rising and LPG dropping. Unless LPG confined in a pit it would no less safe then CNG. The other safety item left out is that CNG is stored in its vessel at 3,000 plus PSI and LPG is stored in its vessel between 50-180 PSI.